Fix build adding a missing variable

app/src/main/java/chat/rocket/android/activity/MainActivity.java

@@ -4,6 +4,7 @@ import android.os.Bundle;
 import android.support.annotation.Nullable;
 import android.support.design.widget.Snackbar;
 import android.support.v4.widget.SlidingPaneLayout;
+import android.support.v7.graphics.drawable.DrawerArrowDrawable;
 import android.view.View;
 
 import chat.rocket.android.LaunchUtil;
@@ -80,6 +81,8 @@ public class MainActivity extends AbstractAuthedActivity implements MainContract
       }
     });
 
+    final DrawerArrowDrawable drawerArrowDrawable = new DrawerArrowDrawable(this);
+    toolbar.setNavigationIcon(drawerArrowDrawable);
     toolbar.setNavigationOnClickListener(view -> {
       if (pane.isSlideable() && !pane.isOpen()) {
         pane.openPane();